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34                        -Bevel Wheel Moulding.

proper radius of arm is -fixed,"by lowering the tooth pattern,
rilling up with sand, raising, rotating the amount of the pitch,
lowering again, and so on until the whole wheel is formed. The.
machine is now taken away by attaching the crane chain to the
eye-bolt at the top of central spindle (see also bottom off. 58).

Core boxes are needed for the wheel boss and segments
between the arms, and for-central hole. Wood strips are used as
gauges to fix the cores in position and to preserve the proper
thickness of the rim and arms; and a cope being placed over
all, gates are formed and the wheel cast.

Bevel wheels are moulded in a similar manner to the
above, the principal alterations being the strickle boards and tooth
patterns. Fig. 48 will make this clear. A board A strikes out
the back of the wheel in green sand, and parting sand is applied;
an impression of the wheel back is then taken in the top box,
and the latter removed to finish; board B next forms bottom face,
cores and boss pattern completing the remainder. The core-
boxes for the wheel arms are shewn in 72^, p. 6t, and similar
boxes will be found in Fig. 42.*

^ Chilled Castings.—Where a very hard and durable surface
is required to a casting, u chilling " is effected by making that
part of the mould, where the said face occurs, of iron. When the
molten metal meets the surface of cold iron, it cools rapidly and
forms crystals of white cast iron, hard yet brittle, where it meets
the iron mould, and, fpr a .depth of* ,an inch gr> rriore within the
casting, according to the mixture used, or' the weight of the chill
mould; the rest of the casting is still grey and soft.

It would seem that the graphite crystals do not, under such
circumstances, have time to form, and so the carbon becomes
! combined with the iron. Suitable objects for chill casting
are:—rolls for plate mills used in forging plates (see Fig. 275,
p. 280), and which require a great depth of chilling, as they
have to be trued up again on being worn down; tram-car
wheels, the rim only being chilled, this class of traffic not
being capable of supporting the expense of steel tyres; points
of plough-shares, which wear away at a very rapid rate in
the earth; bushes for ordinary cart axles; railway chairs; pro-

* For other moulding machines see Appendices II., p. 785, and V., p. 969.